Telecommunication cables are often comprised of a plurality of telesingle wires surrounded by a sheath. The number of telesingle wires may vary from a few in a data transmission cable up to about one thousand in a telephone cable. The sheath surrounding the bundle of telesingle wires consists of at least one layer and may consist of two layers, an inner sheath layer and an outer sheath layer. In order to further protect and isolate the telesingle wires a filler such as petroleum jelly may in e.g. telephone cables be inserted in the voids between the telesingle wires and the sheath. Each telesingle wire normally consists of one solid 0.4-0.5 mm thick copper conductor surrounded by a 0.15-0.25 mm thick insulating layer. The overall thickness of a telesingle wire is thus only about 0.7-1.0 mm.
Another type of data transmission cable, is the so-called coaxial cable, where a central copper conductor, typically from 0.5 up to 2 mm thick, is surrounded by an insulating layer up to 2 mm thick, and then by a coaxial metallic screen which in turn is surrounded by an outer sheath.
The insulating composition of the present invention is intended as the insulating layer of telesingle wires as well as of coaxial cables, but for the sake of simplicity the invention will be explained and illustrated with reference to telesingle wires only. Generally, the properties required of a coaxial cable are substantially the same as those of a telesingle wire.
The insulating layer surrounding each telesingle conductor normally comprises a medium to high density polyethylene composition. The insulating layer may be solid, foamed, or a combination thereof such as foamed with an outer skin or foamed with both an inner and an outer skin. The foam is prepared by introducing a gas such as nitrogen, carbon dioxide, or a solid blowing agent such as e.g. azodicarbonamide (dec. temp. about 200.degree. C.) into the polymer composition. The skin/foam structure is prepared by coextruding the polymer composition in two or three layers and foaming one of the coextruded layers.
Particularly important characteristics of the insulating layer of a telesingle wire are good processability, high thermo-oxidative stability, high environmental stress cracking resistance (ESCR), and good surface finish. The importance of good processability is illustrated by the fact that the copper conductor is coated with the insulating layer in a thickness of only 0.15-0.25 mm at a coating speed of up to about 2500 m/min. In addition the coating must be very even and any exposure of the copper conductor must be avoided because of the risk of short circuiting, overhearing and other signal disturbances. An uneven thickness of the insulating layer also leads to capacitance variations. Further, the telesingle wires of a telecommunication cable are often exposed to very severe temperature conditions and in hot countries the telesingle wires may be exposed to temperatures as high as about 70-90.degree. C. In order to achieve a good thermal resistance various stabilizers lika thermooxidation stabilizers and metal deactivators are normally added to the insulating composition, but such stabilisers are expensive and it would be desirable if the use thereof could be reduced or eliminated. Further still, the fillers such as petroleum jelly and the copper conductor often have a deleterious influence on the insulation, particularly when the telesingle wire is exposed to high temperatures. In order to withstand this deleterious influence the insulating composition should have a high ESCR. Finally, the surface finish of the insulating layer must be high in order to avoid formation of dust when twisting the telesingle wires.
From the above it is evident that the insulating layer of telesingle wires is exposed to a number of very disparate conditions and strains and should display a combination of very specific and to a certain extent contradictory characteristics, particularly with regard to processability, thermo-oxidative stability, and ESCR. An improvement in one or more of these characteristics and a reduction of the amount of stabilisers added would be very desirable and represent an important technical advance.
In this connection it should be mentioned that a bimodal cable-sheathing composition is known through WO 97/03124. This cable-sheathing composition consists of a multimodal olefin polymer mixture, obtained by polymerisation of at least one .alpha.-olefin in more than one stage and having a density of about 0.915-0.955 g/cm.sup.3 and a melt flow rate of about 0.1-3.0 g/10 min, said olefin polymer mixture comprising at least a first and a second olefin polymer, of which the first has a density and a melt flow rate selected from (a) about 0.930-0.975 g/cm.sup.3 and about 50-2000 g/10 min and (b) about 0.88-0.93 g/cm.sup.3 and about 0.01-0.8 g/10 min. It should be stressed that this composition is not an insulating composition for telesingle wires, but a cable-sheathing composition, i.e. a composition for the outer sheathing of a cable, e.g. the sheathing surrounding a bundle of telesingle wires as mentioned previously. The properties required of a cable-sheathing composition are not the same as those of an insulating composition for a telesingle wire. Thus, high mechanical strength and low shrinkage are particularly important to a cable-sheathing, while processability and surface finish are less critical. On the contrary, thermo-oxidative stability, ESCR, and in particular processability are of decisive importance to the insulation of a telesingle wire. These different requirements in properties of a cable-sheathing versus an insulation for a telesingle wire means that a composition optimized for a cable-sheathing would not be useful as an insulation for a telesingle wire and vice versa.